Do you have a yellow gold ring, or perhaps a necklace? If so, it's probably a little better than 50% gold, with other metals to give it hardness against wear. Look at it closely. See how it's the same bright color, even in areas it may have worn over the years?
Now look at your coin. It would be much higher grade gold, close to 100%. It should be bright yellow everywhere on the coin. Gold is valued because it doesn't react with much, even after hundreds of years. If there's a nick or scratch, it should still be bright yellow in the nick.
Are there higher areas of the design that have a different color, usually a gray or brown? If so, your coin is plated gold over base metal, with the base metal showing through where the plating has worn off. If you were to run a zincoln cent over some sandpaper, you'd get the idea--copper color in the lower areas, zinc on the high points.
If your coin is genuine (hint: you're more likely to win the lottery or get struck by lightning) it will be a bright yellow all over. Otherwise, you have one of millions of copies.